Known derailleur systems for multi-speed bicycles include a chain transfer sprocket and a chain take-up sprocket mounted at the end of a parallelogram system, which maintains the two sprockets parallel to themselves when transversely shifted from different positions in alignment with the respective rear drive sprockets. Such a parallelogram arrangement is made of several parts and, consequently, is expensive to make with the required precision. Moreover, such parallelogram devices have to extend on the outside of the bicycle frame and are liable to be hit and broken, especially when used on a mountain bike. The same holds true for the known front derailleur systems used to transfer the chain from one front sprocket to the other. Also, known front derailleurs very often come out of adjustment and fail to properly transfer the drive chain from one front sprocket to the other, since the chain contacting transfer plates of the front derailleur needs to be transversely shifted past the plane of the sprocket onto which it is intended to transfer the drive chain. It is appreciated that this transfer movement has to be carefully adjusted, so as to prevent chain transferring to an unintended sprocket.
Front and back derailleurs are separate entities and are therefore time-consuming to install.
U.S. Pat. No. 4,023,424, dated May 17, 1977, entitled: INBOARD DERAILLEUR FOR MULTI-SPEED BICYCLES--inventors: John W. Ryan and James D. Moore, describes a rear derailleur in which the chain transfer and take-up sprockets are mounted at the end of a rod which is guided for transverse movement by passing through inclined slots made in a pair of vertically-spaced plates secured to the wheel-supporting frame member. Thus, the downward leg of the rod is maintained vertical to maintain the two sprockets in a vertical plane. Substantial force is required to be applied to the longitudinal rod, in order to effect chain transfer. This force would exceed the normal capacity of the remotely-actuated flexible cable actuators.